1. Field of the Invention
The invention relates to combining multiple images of a biological structure, particularly to determining electrical properties (e.g., resistivities and/or conductivities) of the biological structure from multiple images of the biological structure.
2. Description of the Related Art
The advent of transcranially stimulated electrical motor evoked potentials (tcMEPs) has resulted in a dramatic reduction in the rate of paralysis for high risk surgical patients (see Chappa K H, 1994, Calanchie et al 2001, Pelosi et al. 2002, Bose B, Sestokas A K, Swartz D M 2004 and MacDonald et al 2003, citations below and hereby incorporated by reference). As a consequence tcMEPs have become the standard of care for testing the integrity of the cortical spinal track during spinal and neurosurgical procedures. Unfortunately, transcranial electrical stimulation has generally required high voltages with diffuse current spread that causes the activation of large regions of the brain and puts the patient at risk of unwanted and unknown side effects. Obtaining more precisely directed current at lower voltages will reduce the risk and greatly expand the utility of transcranial stimulation for surgical and non-surgical patents.
It is desired to have a technique involving site specific transcranial electrical stimulation of the brain that approximates physiological current densities, and to apply these techniques to treat expanded patient populations, including spinal surgery patients. Transcranial electrical stimulation to elicit motor evoked potentials (tcMEPs) has become the standard of care for monitoring the motor pathways of the spinal cord and brain during high risk surgeries. A conventional tcMEP technique can often be a crude, but effective tool to monitor motor pathways and to identify iatrogenic injuries. FIG. 1A illustrates a tcMEP from a scoliosis patient. The scale of FIG. 1A shows 50 μV on the y-axis and 7.5 ms on the x-axis. Applied pulses were 150 Volts for 100 μs in trains of five pulses with an inter stimulus interval (ISI) of 3 ms. FIG. 1B illustrates a tcMEP from a 86 year old male with a neck fracture. Applied pulses were 75 Volts in the upper plot and 25 Volts in the lower plot.
Typically, a tcMEPs procedure involves placing electrodes in the patient's scalp at locations that are thought to encompass the motor cortex and then applying brief high voltage electrical pulses with the intention of activating distal muscles or muscle groups. FIG. 2 illustrates placement of electrodes J0 outside of a patient's scalp. FIG. 2 also illustrates three regions S0, S1, and S2 having different conductivities σ1, σ2, and σ3, respectively. Unfortunately, the high voltages typically used to induce tcMEPs and the responses they produce can activate whole regions of the head, body, or trunk as well as the target muscles. The movement of large muscle groups due to the uncontrolled current spread means that seizures, broken jaws and patient movement create risk factors that have been associated with tcMEP testing (see Chappa, K H, 1994, citation below). Applying stimulus trains rather than single pulses and adjustments in anesthesia techniques have significantly reduced the applied electrical currents used from 700-900 V to 200-400 V (see Chappa, K H.1994, Haghighi S S, and Zhange R 2004, citations below and hereby incorporated by reference).
TcMEPs have become widely accepted as a less onerous substitute for “wake-up tests” in which the patient is awakened during surgery and asked to move their limbs before the surgical procedure is completed (see Eroglu, A et al. 2003, citation below and hereby incorporated by reference). However, these reduced stimulus levels still exceed normal physiological levels and the uncontrolled movement of large muscle groups suggests that the applied pulses continue to result in significant current spreads. While major side effects are relatively rare, tongue lacerations, muscle tears, and bucking are still rather common side effects (see Calanchie, B et al. 2001, citation below and hereby incorporated by reference). The large muscle movements that are sometimes associated with tcMEPs also limit the usefulness of the tcMEPs during periods when the surgeon is involved in delicate brain or spinal procedures.
It is desired to reduce or eliminate these side effects by predicting the paths of electrical pulses within the brain and consequently adjusting current levels (i.e., lower). It is also desired to reduce the current strength to near physiological levels at targeted areas to allow brain electrical stimulation to be used for treatment of patients outside of surgery. In this way, a significant positive impact on the treatment of a number of disease conditions that have been demonstrated to benefit from brain electrical stimulation, e.g., Parkinson's disease, chronic pain, and depression, can be achieved.